Geomancer

[Archwizard]

We write it off because it makes zero design sense to have two similar Jobs in the same role. And SE supports our theory just by their history of Job additions. Look at the list of what we have now and most would agree none of them have any thematic overlap.

Arguably there's some thematic overlap for specific jobs (like, say, Gunbreaker and Machinist's use of technology, Dark Knight and Warrior's use of their inner rage, or Dancer and Bard's support through entertainment), but significant enough difference in their mechanics and/or role that they each carve their niche.

By contrast though, Geomancer simply drawing energy from the land would be exactly the same as a Conjurer within the healing role and would have difficulty manifesting in ways we haven't seen among existing healers -- but would be unique in a role like caster DPS.

Dark Knight? Samurai? Gunbreaker? I mean, you can throw a dart at the board of jobs and chances are a noticeable amount of people called the job as something other than what we got.

... And I would argue most of whom were pegged as one of two roles, and were often denied others that fell outside their niche.
To use your example, jobs like Dark Knight, Samurai and Gunblade (before Gunbreaker was established) were always going to be argued as either melee DPS or tank -- because any new job introduced is always going to lean towards either damage or support. The only jobs that are iconically a single fixed role within the holy trinity are White Mage, Black Mage, and Paladin, who were quite obviously the first ones put on the plate.

But I would also point out that even by dint of your own argument, if Geomancer were to be added, we're as likely to see it be literally any other role including one that doesn't exist yet as we are to see it be a healer -- essentially throwing up your hands and saying "well screw it, why not make it anything then" -- which makes the discussion point non-constructive within the topic, if not even self-defeating as an actual point.